For example, JP-A-11-139225 discloses a tunnel detection apparatus provided in a vehicle, for detecting a tunnel according to the traveling of the vehicle. Specifically, a setting density region detection unit detects a region of specified density from a forward road image photographed by a photographing unit. Because a tunnel is photographed under an illumination condition different from a surrounding environment, the inlet of the tunnel or the outlet thereof appears as a different density region on the image. Accordingly, when those are detected as specified density regions, the inlet or the outlet region indicating the existence of the tunnel is detected.
In addition, JP-A-2001-39210 discloses a technique in which before a vehicle enters a dark place such as a tunnel, the light of the vehicle is suitably automatically switched on to improve the sense of use, convenience and safety. Specifically, a lighting control apparatus automatically switches on or off the light of the vehicle according to an illumination measured value by an illuminometer, and includes a video camera for taking a photograph in a traveling direction, a dark part detection unit, which calculates the occupying ratio of a dark portion in the image data of a forward view image obtained by the video camera and outputs a dark portion detection notice in the case where the occupying ratio exceeds a predetermined first dark portion threshold, and a light control unit to automatically switch on the light in preference to the lighting control based on the illumination measured value of the illuminometer when the dark portion detection notice is received from the dark portion detection unit. Accordingly, even when the vehicle travels in the daytime and in bright environment, in the case where a dark place such as a tunnel exists in the traveling direction, sufficiently before the vehicle enters the dark place, the light is automatically switched on.
Furthermore, JP-A-9-205645 discloses a forward environment recognition apparatus in which it is possible to prevent an image signal value from becoming excessively small or excessively large in total when a vehicle enters or comes out of a tunnel, and an image processing can be always certainly carried out based on the suitable image signal. Specifically, in a case where an image pattern of an inlet of a tunnel is previously stored in a memory of an image processing unit, based on the pattern matching between the image pattern and an image object over a road white line, it is confirmed that this is the inlet of the tunnel. Then, based on the number of pixels occupying under the inlet of the tunnel, a distance between the traveling vehicle and the inlet of the tunnel is calculated. Besides, in a case where iris value data (inlet iris map) set for each distance to the inlet of the tunnel is previously stored in the memory, when the vehicle approaches a place distant from the inlet of the tunnel by a predetermined distance, normal iris value control (auto iris control) is gradually changed to iris value control in the tunnel by a control unit, and the iris opening of a CCD camera is controlled based on the iris value data corresponding to each distance to the inlet of the tunnel. As a result, it is possible to prevent the image signal value from becoming excessively small in total when the vehicle enters the tunnel, and the image processing can be certainly carried out based on the suitable image signal.
Moreover, JP-A-2004-230963 discloses a vehicular tunnel detection method in which even in the case where a preceding vehicle exists, the inlet of a tunnel can be certainly detected, and the cost can be reduced. Specifically, in an image processing unit (tunnel detection unit), in the case where a tunnel recognition unit using a distance judges that the center portion (in the tunnel) is farther than both right and left side portions (right and left wall surfaces of the inlet of the tunnel) of an image A, and a tunnel recognition unit using luminance judges that the luminance of the center portion (in the tunnel) is darker than those of both the right and left side portions (right and left wall surfaces of the tunnel inlet), the inlet of the tunnel is detected.
In the conventional techniques described above, the tunnel is detected by using the difference in the lightness between the inlet and the outlet of the tunnel, the tunnel is detected by previously preparing the image pattern of the inlet of the tunnel and by carrying out the matching, or the tunnel is detected by using the stereo camera to obtain the distance information and by utilizing a fact that only the tunnel inlet portion has a large distant. In the tunnel detection methods of the conventional techniques as stated above, there is a problem that because the detection of the tunnel is carried out by using the feature of the inlet or the outlet of the tunnel, the inlet or the outlet of the tunnel cannot be detected due to the existence of a preceding vehicle or in the night, and it is impossible to discriminate whether the vehicle travels inside the tunnel or outside the tunnel.